F(x) = Y = x^3 - 7x^2 + 25x - 39 = 0.
The real solutions to the Eq are the x-intercepts or the points where the graph crosses the x-axis. The value of Y
where the curve crosses the x-axis is 0.
That is why Y is set to 0.

All real values of X that satisfy the Eq
is a real solution. It was found by trial and error that 3 gives a zero output(3,0).

X = 3
x-3 = 0 and it is a factor.
Using long-hand division, we divide the
Eq by x-3 and get x^2-4x+13. Now we have(x-3)(x^2-4x+13) = 0